Clean it REAL good. It's not uncommon for the lube to form skinny little lines of grit and lube that mark and stain the hub. If you've cleaned up real nice and you still suspect it is cracked you can have it dye penetrant inspected to verify, or just replace it if that's not cost effective, or if you are certain it is cracked. I'd think that a cracked one would be pretty rare unless you are putting a lot of shock to it on a regular basis, but since it's a cush drive, it should even take drag race start type loads for a long time with no ill effects.